Afferent neural activity arising from peripheral mechanoreceptors monitoring cardiopulmonary function not only produces well-characterized CNS-mediated reflex adjustments of the circulation, but also activates CNS systems that modulate noxious somatosensory input. The present proposal is based upon the hypothesis that peripheral and central substrates of baroreceptor reflex control of the circulation are physiologically linked to systems involved in the centrifugal modulation of pain. Thus, the primary objective of this proposal is to parametrically evaluate the modulation of spontaneous and noxious-evoked activity of spinal dorsal horn neurons by manipulations which increase afferent activity of the vagal nerve trunk in the rat. This will be accomplished by extracellular recording of antidromically- identified class 2 ("wide dynamic range") and class 3 ("nociceptive specific") neurons of the lumbar spinal dorsal horn activated by either tibial nerve stimulation (A-fiber and A- and C-fiber intensities) or noxious heat stimulation applied to the plantar surface of the hindlimb footpad. Activation of vagal afferents will be effected by two manipulations established as antinociceptive in the rat: electrical stimulation of the vagal trunk or intravenous administration of (D-ala2)- methionine enkephalinamide. The specific aims of the present proposal are: (1) to provide converging electrophysiological support for existing behavioral data on cardiopulmonary modulation of somatosensory input in the rat; (2) to evaluate the organization of medullary substrates mediating inhibition of spinal nociceptive transmission produced by activation of vagal afferents; and (3) assess potential interactions between known medullary substrates of descending spinal inhibition and those activated by vagal afferents.